Over the years electrostatic and laser color copy/printers have shown significant improvement in their ability to make copies or prints giving excellent color rendition and image quality. The new generation of copiers and printers are now able to produce prints having quality comparable to that of silver halide color systems. With the advent of this ability the industry has attempted but has failed to produce, through electrostatic processes, images that have the look and feel of silver halide prints.
Plain paper is typically used in electrostatic printing applications which does not generally provide a high degree of resolution, especially when color is involved. In photographic applications resin coated papers are used to provide the necessary resolution and quality. However, use of resin coated paper, i.e. polyethylene resin, as a copy or printing media in electrostatic printing applications has been a problem. Typical fuser roll temperatures are between 125 to 225.degree. C. Due to the low Tg of the resin the polyethylene softens or melts when coming into contact with the toner fuser roller of the copier/printer. This softening or melting causes paper jams and image degradation.
In addition to the "melt problem", electrostatic imaging directly on a resin coated paper substrate has been a problem. This is due to the toner used in electrostatic processes which is generally incompatible with the resin coated paper. Thus transfer and adhesion of the toner particles to the resin surface is not satisfactory and compromises the image production.
Thus in electrostatic imaging applications to have the feel and look of a standard silver halide print plain paper can not be used and use of "photographic type" substrates such as polyethylene resin or similar coated substrates are inadequate and pose a problem of softening or melting on the fuser roller. Accordingly, there is a need in the art for a resin coated paper for use in electrostatic applications to overcome these problems.
The invention provides such a solution by coating a coating layer over the resin coated substrate i.e. polyethylene layer. This coating layer typically comprises a natural or synthetic film forming polymer that has a melting point above 140.degree. C., thus preventing the resin coated substrate from melting and sticking to the fuser roller.
This protective layer provides a receiving surface resulting in photorealistic quality prints or copies. Depending on the optional ingredients in the layer a high gloss or semi-matte finish can be created. The invention provides advantage over conventional copying processes as well as over conventional photographic developing processes by providing an environmentally friendly process of producing photorealistic quality prints or copies without using toxic chemicals.
Generally, resin coated papers and adhesive-like gelatin "subbing" layers used for receiving an image are known in photographic applications.
U.S. Pat. Nos. 3,811,913 and 4,188,220 to Kasugai et al. are representative of a "subbing" layer and resin coated paper, respectively, for use in photographic film processing. The '913 patent discloses use of gelatin and other polymeric materials as "subbing" layers for photographic materials including polyethylene coated paper substrates. UV radiation is applied to the coated surface to improve the adhesive property of the polyethylene support to the subbing layer. The '220 patent discloses a polyolefin coated paper. A low molecular weight polyolefin resin is incorporated into a conventional polyolefin resin to provide a coating layer.
U.S. Pat. No. 4,312,937 to Kasper et al. discloses a resin coated paper including a paper layer and first and second layers of polyolefin adhered to opposite sides of the paper layer. Carbon black is incorporated into the polyolefin layers to eliminate pin-holing.
U.S. Pat. No. 4,547,445 to Asahina et al. discloses a photographic material ("postcard") capable of having a photograph on one side and a writing surface of the opposite side. A paper support is coated with a polyolefin resin on both surfaces. A photographic emulsion layer is coated on one surface of the support and the opposite surface is coated with a gelatin layer including an inorganic pigment to absorb inks.
U.S. Pat. No. 5,055,320 to Miura et al. discloses a support sheet including a subbing layer and a photographic emulsion layer. U.S. Pat. No. 5,075,196 to Daems et al. discloses supports for halftone dot image production. Daems provides a process including a paper base support coated on at least one surface with a polyolefin layer. On the exterior of the polyolefin layer is a white pigmented binder layer comprising a hydrophilic colloid binding agent and white titanium dioxide pigment particles. The light sensitive layer is coated on top of this binder layer.
U.S. Pat. No. 5,082,724 to Katsura et al. discloses photographic paper supports consisting of a base paper support coated on both sides with a polyolefin resin.
U.S. Pat. No. 5,104,721 to Sun discloses an electrophotographic imaging media comprised of a polymeric coating on a film substrate (slide projections) to improve printing resolution. The polymeric coating contains at least one pigment and has a Tukon hardness of from about 0.5 to about 5.0 and a glass transition temperature of from about 5 to 45.degree. C.
U.S. Pat. No. 5,141,599 to Jahn et al. discloses a receiving material for ink jet printing including a polyolefin coated base paper with an ink receiving layer applied on the top surface. This receiving layer includes a mixture of gelatin and starch. The receiving material is defined as a gloss surface for ink jet printing comprising a polyolefin coated base paper and an ink receiving layer. The ink receiving layer contains a mixture of gelatin and starch in a ratio of 1:1 to 10:1 with the starch of a specific grain size.
U.S. Pat. No. 5,182,161 to Noda et al., discloses a "support for photosensitive materials" comprising a base paper formed from natural kraft pulp according to a specifically defined digestion and chlorine bleaching process and a resin layer formed on the base paper. A subbing layer comprising a hydrophilic polymer such as gelatin is formed on the resin layer.
In addition the following Japanese patents all relate to papers suitable as "photographic supports" comprising paper coated on at least one side with a polyolefin and with one polyolefin surface over-coated with a hardener-containing gelatin layer. Japanese Patent No. 50-19402 dated Jun. 20, 1973; Japanese Patent No. 50-61154 dated Sep. 28, 1973; Japanese Patent No. 50-66519 dated Oct. 9, 1974; Japanese Patent No. 60-64306 dated Apr. 12, 1985; Japanese Patent No. 61-84643 dated Apr. 30, 1986; Japanese Patent No. 1-137252 dated May 30, 1989; and Japanese Patent No. 2-849 dated Jan. 5, 1990.
It is seen that gelatin over-coated polyethylene coated paper substrates in photographic applications is known. The Japanese patent abstracts describe such papers suitable as photographic supports, however, unlike the invention the gelatin layer is used as an undercoat on which a photographic emulsion coating is applied. This latter emulsion coating described in the prior art is the layer in which the image is formed by processing in an aqueous developing solution. These adhesive-like gelatin "subbing layers" are also described in Kasugai '913; Miura and Noda and also include a photographic emulsion coating thereon. Gelatin layers coated over polyolefin resin coated paper supports are described in Asahina et al. and Daems et al. However the exterior gelatin layer in Asahina includes an inorganic pigment and in Daems includes a white pigmented binder. The gelatin binder layer in Daems is further coated with a "light sensitive" layer.
Kasugai '220 and Kasper are representative of polyolefin coated papers. However, in contrast to the present invention, the resin layer in Kasugai '220 includes a low MW polyolefin resin and in Kasper includes carbon black. Both Katsura et al, and Sun disclose in general only "polyolefin coated" substrates: Kasura defines specific pulp fibers used to produce the base paper which is then coated with polyolefin; and Sun discloses a polymeric (film) substrate coated with polyolefin. Finally, Jahn discloses an ink jet sheet including a receiving layer which is a mixture of gelatin and starch.
The prior art does not teach a coated paper stock for electrostatic imaging. The present invention is directed to the provision of such by providing a substrate coated on at least one surface with a resin layer and a coating layer over the resin layer comprised of one or more natural or synthetic film forming polymers and has a glass transition temperature above 100.degree. C. The particular combination of resin coated paper of the invention provides an electrostatic copy or printing medium that is heat resistant. The coating layer of the invention also provides an image receiving surface layer in which toner particles are transferred and adhered to the surface during electrostatic imaging processes to produce photographic quality prints or copies.
It would be appreciated that advantage over known applications would be obtained by providing a coated paper where the melting point of the resin coating remains greater than 140.degree. C., preferably greater than 200.degree. C., to avoid problems of melting and toner adhesion during electrostatic imaging applications. In addition, this "outer" image layer provides a "hard" surface, which in a preferred embodiment comprises a gelatin and a crosslinking component. This is in contrast to the gelatin layers shown in the prior art which are soft and used as "adhesive" subbing layers. The "hardness" property is desired in the invention since the gelatin layer itself is used as the toner receiving surface layer.
Accordingly, it is a broad object of the invention to provide a coated paper stock for electrostatic imaging comprised of a resin coated substrate with a coating layer.
A more specific object of the invention is to provide a coated paper stock for electrostatic imaging where the "outermost layer" provides heat protection, gloss control, image improvement, improved smoothness, and improved toner adhesion and transport within the electrostatic imaging apparatus.
Another more specific object of the invention is to provide a coated paper stock for electrostatic imaging including a substrate coated on at least one surface with a resin layer comprised of olefinic material and an outer most heat protective layer, where the heat protective layer is a pin-hole free, continuous film over the resin layer.
Another object of the invention is to provide a coated paper stock and related process for producing a 3-dimensional relief image in an electrostatic imaging apparatus.
Another object of the invention is to provide a method for manufacturing a coated paper stock for electrostatic imaging.
A specific object of the invention is to provide a dry method for producing photorealistic quality prints or copies which is advantageous over conventional photographic developing processes by being environmentally friendly and not using toxic chemicals.
A more specific object of the invention is to provide a method for using a coated paper stock in electrostatic imaging applications without the associated problems of melting, image degradation and toner incompatibility.